Willie is disappointed after his first foot race because?

Willie lost the race to Sam...... Uncle Tom's dog.

Willie shot forth, half running, half stumbling. He clenched his fists even tighter. Bang! He fell with a hard thud onto his knees. Pushing himself up, he staggered on, feeling angry and desperate inside. In his heart he wanted to run properly, but his stupid legs were letting him down. He heard Sam barking behind him.
"Go it," shouted Tom. "Go on, William!" and, before he realized what he was doing, he was running too.
Willie propped himself up against the gate gasping for breath while Sam sat nonchalantly by his feet, an easy winner.